Islamic State published a video on Sunday threateningEgyptian Christians and showing the last statement of a man it said was responsible for the deadly bombing in December of a Coptic cathedral in Cairo.

The masked man in battle-dress, whom the group identified as Abu Abdallah al-Masri, is seen encouraging militants all over the world not to give up and promising Islamists jailed in Egypt they will be freed soon, when the group "liberates" Cairo.

President Abdel Fattah al-Sisi had already identified the bomber as a 22-year-old student called Mahmoud Shafik, and it is believed Abu Abdallah al-Masri was his assumed name. He had been detained for two months in 2014 before joining an Islamic State cell in Sinai, the government said.

"Finally, to my brothers in captivity: rejoice, you believers, do not falter or grieve. I swear to God we will very soon liberate Cairo and free you from captivity. We will come bearing explosives, I swear we will, so rejoice you believers," he said in the video.

At least 28 people, mostly women and children, were killed in December when a bomb exploded in a chapel adjoining St Mark's Cathedral, the seat of the Coptic papacy.

Orthodox Copts, who comprise about 10 percent of Egypt's 90 million people, are the Middle East's largest Christian community.

Islamic State claimed responsibility for the bombing, its deadliest attack in Egyptoutside the Sinai Peninsula, where it has been waging an insurgency since 2013.

The Cairo attack, coupled with the group's use of a new "Islamic State in Egypt" logo as opposed to "Sinai Province", suggests an expansion of operations to the rest of the country.

The Egyptian government said in December that the bomber supported the Islamist Muslim Brotherhood movement. But the Brotherhood condemned the attack and accused Sisi's administration of failing to protect the church.

Sisi took power in 2013, deposing the Brotherhood's President Mohamed Mursi, and has since outlawed the group as part of a crackdown in which hundreds of Islamists were killed and thousands jailed.

The Brotherhood says it is peaceful movement but it has split into rival wings, and some erstwhile supporters have formed splinter groups which carry out attacks on police and judicial officials.

Disillusioned by its ill-fated flirtation with democracy, some younger Brotherhood supporters have gone to fight in Syria or joined the local arm of Islamic State, which since 2013 has killed hundreds of soldiers and police in Sina